Contaminated water supplies have long been recognized as a problem and the interest has been heightened by population movements and public awareness of the effects of pollution which attend an industrialized society.
The sources of contaminants which may enter aquifers are many and may be man-made or naturally occurring. Based upon analysis of volume and relative toxicity, four categories of pollutants are most frequently addressed. Organic solvents including chlorinated cleaning fluids such as TCE, liquid fuels such as gasoline and heating oil, toxic metals of industrial and geological origin and agricultural chemicals, especially herbicides.
Chemical solvents and refined petroleum products typically have a significant vapor pressure and are characterized by the environmental protection agency as volatile organic compounds (VOC). Most are sparingly soluble in water and can be stripped at atmospheric pressure or below by sparging. They behave according to Henry's Law. Sparging is a well-known technique and has been practiced in various forms for decades, if not centuries. Numerous patents have taken advantage of Henry's Law to purify water, including contaminated aquifers.
U.S. Pat. No. 4,832,122 to Corey et al. discloses a two well system operated in a push-pull fashion to force volatiles from contaminated groundwater.
U.S. Pat. No. 5,104,554 to Dempsey discloses the use of a sparger to modify a conventional drinking well for the purpose of removing radon gas.
U.S. Pat. No. 5,180,503 to Gorelick et al. teaches stripping VOC's from a saturated zone using compressed air. The water entrained by the rising air is returned to the vadose zone via a plurality of buried pipes.
Pennington, U.S. Pat. No. 5,425,598, describes a sparging apparatus which uses air lift to sparge contaminated groundwater and create a circulation of water from a lower level of an aquifer to a higher level of the same aquifer.
U.S. Pat. No. 5,577,558 to Abdul et al. is directed to sparging and air lifting contaminated groundwater as part of a treatment system which includes injection of compressed gases.
U.S. Pat. No. 5,622,450 to Grant, Jr., uses sparging and gas lift as part of a method for treatment using gas injected into the vadose zone to support aerobic decomposition of contaminants and to vent the vadose zone.
Organic liquids which have a low vapor pressure are not readily sparged and the energy required to separate them from water by steam distillation is prohibitively expensive when large volumes of liquid are under consideration. Adsorption onto a solid substraight strate is one alternative for their removal.
Toxic metals occur in several forms. Zero-valance metals are usually insoluble in water but may be carried in suspension as particles. Metal salts, which may have been introduced in kind into the aquifer or which may have been formed in an aquifer having a low pH are not removed by sparging but are amenable to treatment using ion exchange resins. Organometallic compounds constitute a less common problem and must be addressed on a case-by-case basis.
Persistent, low volatility, solid organic compounds migrate slowly but their toxicities tend to be cumulative. This is particularly true of chlorinated and brominated compounds used as herbicides, moldicides and nematocides. Treatment is species specific and advantage is often taken of the polarity of the compound in selecting a suitable treatment agent.
The purification of water in small volumes is not a daunting task chemically, but the economical treatment of large volumes of water at times is challenging.
Pumping water out of an aquifer and treating it above ground has been disclosed in many patents.
Examples include U.S. Pat. No. 5,358,357 to Mancini et al. and the aforementioned patent to Grant, Jr.
When a contaminated aquifer is pumped to the surface for treatment, facilities must be provided for the treatment and the cost of construction may be significant. The above-ground facilities are not easily disguised and require regular maintenance. The treated water must be returned to the source by either a second well or by spraying over a large drainage field, especially in residential neighborhoods where leaking buried fuel oil tanks threaten nearby wells. Unsightly above-ground treatment facilities may lead to social unrest. Discharge permits may be required under national or local laws.
In situ remediation to remove low volatility compounds has been attempted. U.S. Pat. No. 5,398,756 to Brodsky et al. and U.S. Pat. No. 5,476,992 to Ho et al. are typical of methods which treat by addition of a foreign material into the contaminated region. U.S. Pat. No. 5,534,154 to Gillham describes methods for the treatment of groundwater in situ using a trench or externally. The specific method of treatment is a combination of activated carbon and "metals" typically including iron.
Abdul et al. cited above uses microbes is a treatment region contained within a wellbore and located below ground level.
Geological formations occur in an almost-infinite variety and consist of various layers of loam, clay, shell and rock. A multiplicity of aquifers may exist at different depths and their plan areas may or may not largely overlap. When pumping an aquifer for any reason, it is desirable to return as much water as possible to maintain hydraulic pressure.